In recent years, the higher strength of steel sheets used in various components and structures, such as automotive outside sheets (body sheets), construction machines, and, in addition, building and civil engineering structures, has been increasingly demanded, and a high-strength steel sheet with a maximum tensile stress of 900 MPa or more has also been used. Further, the steel sheets having such uses usually need excellent corrosion resistance because of often being used outdoors.
Conventionally, as steel sheets having such uses, hot dip galvanized steel sheets subjected to hot dip galvanizing have been widely used. Recently, there a galvannealed steel sheet subjected to alloying treatment including hot dip galvanizing, thereafter heating a plated layer to a temperature that is not less than the melting point of Zn to diffuse Fe from a base steel sheet into the plated layer, and forming the plated layer as a layer based on a Zn—Fe alloy has also been widely used. Such a galvannealed steel sheet is known to have excellent surface appearance and corrosion resistance, compared with a hot dip galvanized steel sheet that is not subjected to the alloying treatment.
However, in uses for automotive outside sheets and the like, the periphery of a sheet is usually subjected to severe bending working (hemming) by press working; and, in not only the automotive outside sheets but also in other uses, such a sheet, subjected to severe bending working, bore-expanding working, or the like by press working, is often used. In addition, when a conventional galvannealed steel sheet is subjected to severe bending working, bore-expanding working, or the like, a plated layer might peel from a base steel sheet in a portion worked in such a manner. When the plated layer peels off in such a manner, there is a problem that the corrosion resistance of an area where the plated layer peels off is lost to early corrode and rust the base steel sheet. Even when the plated layer does not peel off, lose of adhesiveness between the plated layer and the base steel sheet to produce even a few voids in an area where the adhesiveness is lost causes outside air and moisture to enter the voids, the anticorrosion function of the plated layer to be lost, and the base steel sheet to be early corroded and rusted in the same manner as described above. Thus, there has been a strong desire to develop a galvannealed layer and a plated steel sheet, having the excellent adhesiveness of the plated layer with a base steel sheet, for uses in which such severe bending working or the like is performed.
There have already been proposed various ways for improving the adhesiveness of a plated layer with a base steel sheet in a galvannealed steel sheet, and some examples thereof are described in Patent Literatures 1 to 8.